1. Field of the Invention
The invention relates to the process and the products obtained by a partial removal of carbon from coal ash which is used in the production of concrete. The carbon removed, through a thermal process of Partial Carbon Burn-Out (PCBO) can no longer reduce the activity of air entraining agents, which increase the stability, the number and the size of air voids in the concrete. The presence of appropriate air voids improves the freeze-thaw resistance of concrete.
2. Description of the Prior Art
Concrete consists of a cement binder, a pozzolanic material such as fly ash which will react with a mineral alkali, an aggregate material and sufficient water to cause the cement to set and bind the components into a hardened and durable material. A pozzolanic material reacts with calcium hydroxide, a by-product of Portland cement hydration to form compounds having the cementitious properties.
Unburned carbon in fly ash has a broad particle size distribution ranging from coarse char ( greater than 75 xcexcm) to very fine soot sized (=40 nm) amorphous carbon particles that has a very high surface area (Gao, Y. M., H. S. Shim, R. H. Hurt, E. M. Suuberg, N. Yang, xe2x80x9cEffects of Carbon on Air Entrainment in Fly Ash Concrete: Role of Soot and Carbon Blackxe2x80x9d, Energy and Fuels 11, 457, 1997). The very fine unburned carbon in fly ash has properties similar to that of activated carbon and as such has an affinity for molecules that have hydrophobic moieties, such as air entraining agents.
The durability of concrete to freeze-thaw cycles is dependent on its level of air entrainment. When water freezes its volume increases 9%. If concrete is fully saturated, all air voids are filled with water, the tensile stresses generated by the freezing water are sufficient to cause cracking and deterioration. Concrete has excellent strength in compression but its tensile strength is approximately 7%-10% of the compressive strength. When concrete begins to freeze the expanding ice forces water into the unfrozen regions of the cement binderxe2x80x94this movement of water creates large hydraulic pressures and generates tensile stress.
Many factors affect the durability of concrete to cycles of freezing and thawing: the cement binder content; amount and type of pozzolan; water to cement ratio; quality of the aggregates; and, the presence of air voids with an optimum spacing and size distribution.
Air is naturally entrapped in concrete through the folding and shearing action of mixing the cement paste. The entrapped air voids are large and not stable in concrete unless air entraining agents are used. Air entraining agents are surface active agents or surfactants. These agents reduce the surface tension of water, which tends to promote the fragmentation of large air voids into smaller ones, and to stabilize air voids by collecting at the water interface and forming a film.
The unburned carbon residue present in fly ash has a high adsorptive capacity for air entraining agents. The time dependent adsorption of air entraining agents causes a loss of entrained air during mixing and placement, and ultimately affects durability of the concrete by degrading the air void system.
LOI (Loss On Ignition) is a measure of the residual combustible material, primarily carbon in the coal ashes. There are several processes in commercial use that aim to reduce the LOI of moderate to high LOI fly ashes significantly to a level below 3% by weight. These include methods employing electrostatic separation or by carbon combustion. It should be noted that carbon makes up most of the measured LOI (to within about 10%) but more particularly it is the adsorption capacity of fly ash for air entraining agents and not the LOI, that is the ultimate criteria for whether the fly ash is commercially useful.
U.S. Pat. No. 5,399,194 describes a thermal treatment for fly ash in a fluid bed between 800 and 1300xc2x0 F., or (426 and 700xc2x0 C.) while U.S. Pat. No. 5,160,539 describes the use of a fluid bed to reduce the LOI in a temperature range from 1300 to 1800xc2x0 F. or 700 to 982xc2x0 C. and is clearly designed to eliminate as much carbon as possible.
One object of the invention is to provide a process for producing a coal ash having a low adsorption affinity for air entraining agents in concrete. This process comprising a partial combustion of a carbon residue of the feed coal ash wherein a fine carbon fraction of said carbon residue that is responsible for adsorption of air entraining agents in concrete is combusted, while leaving a non-combusted carbon fraction with a lower adsorption capacity for air entraining agents; and a recovery of the coal ash product.
It is another object of this invention to provide a method for inhibiting adsorption characteristics of coal ash bearing a carbon residue. The method comprising combusting an adsorbing fraction of said carbon residue responsible for adsorption of air entraining agents in concrete, while leaving non-adsorbing carbon of said carbon residue un-combusted in said coal ash.
Another object of the invention is a coal ash product which has low levels of a fine carbon fraction responsible for adsorbing air entraining agents in concrete. The fly ash product for use in concrete, is produced by a partial combustion of a carbon residue of a feed coal ash wherein the fine carbon fraction of said carbon residue responsible for adsorption of air entraining agents in concrete is combusted, while leaving a non-combusted carbon fraction of non-air entrainment agent absorbing carbon in the coal ash product.
Yet another object of the invention is a concrete product. The concrete product having a cement binder, an aggregate, and an air entraining agent characterized in that it includes, a coal ash product which has low levels of a fine carbon fraction responsible for adsorbing air entraining agents in concrete produced by, a partial combustion of a carbon residue of a coal ash wherein the fine carbon fraction of said carbon residue responsible for adsorption of air entraining agents in concrete is combusted, while leaving a non-combusted carbon fraction of non-air entrainment agent absorbing carbon in the coal ash product, which are all mixed together with water to produce said concrete product.
When concrete contains sufficient cement ( greater than 225 kg/m3) and the water to cement ratio is less than 0.55:1, the concrete will be durable providing that an air entraining agent is used to generate an air void system consisting of very fine bubbles (50 to 200 xcexcm). The average distance from any point in the cement paste or the spacing factor, must not be more than 200 xcexcm from the nearest air void in order to protect the cement paste. The air voids in air entrained concrete relieve the pressure (i.e. tensile stresses) by accommodating the hydraulic pressure produced by the expansion of water at lower temperatures and prevent damage to the concrete. Air entrained concrete that has adequate strength and is appropriately saturated with air will withstand hundreds of freezing and thawing cycles without dilating or losing strength.
Air entraining agents promote the adhesion of air voids to the surface of hydrating cement grains which further stabilizes the air void in the cement paste. In general, as the dosage of the air entraining agent increases, the air content increases up to a maximum value, after which additional increases in air entraining agent do not increase air content significantly. The dosage rate of air entraining agent required to achieve a target air content is dependent on the type of air entraining agent, concrete temperature, concrete materials, such as sand, cement, fly ash, and mixture proportions.
The adsorption of air entraining agents by the carbon contained in coal ashes, reduces their concentration in solution, which in turn causes the destabilization of air voids. Under such conditions air voids coalesce into larger voids, with some rising to the surface and being lost. The remaining fewer and coarser air voids are much less effective in protecting the concrete because of their low surface area.
Regardless of the actual mechanism of air entrainment, a certain concentration of air entraining agent is required to achieve an air content of 5% to 8% in concrete. Normal cement hydration absorbs a portion of the air entraining agent, which causes the air content of the concrete to decline with time.
There are various air entraining agents available and may contain a metal cation and an non-polar organic group. Typical air entraining agents include: neutralized Vinsol resin, sodium abietate, sodium oleate and sodium dodecylbenzene sulfonate.
The ASTM C618-01, test method for the xe2x80x9cStandard Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use as a Mineral Admixture in Concretexe2x80x9d, stipulates for both Class C and Class F fly ashes a maximum Loss-On-Ignition (LOI) of 6%. Experience has shown that with certain fly ashes, the entrainment and maintenance of an air void system is difficult and in some cases impossible, despite meeting the requirements of ASTM C618-01. Though Class C fly ashes have much less unburned carbon than Class F fly ashes they can also have difficulties entraining and maintaining an air void system. Class C fly ash is typically produced from lignite and sub-bituminous coal and contain higher quantities of lime (CaO), which in addition to having pozzolanic properties gives Class C fly ash cementitious properties as well. Class F fly ash is produced from the combustion of bituminous and anthracite coals, with far lower lime levels and little to no cementitious properties.
Unburned carbon in fly ash is either amorphous or crystalline, and the relative amounts of the two depend on the coal and combustion conditions in the boiler. Both types of carbon absorb air entraining agents but the amorphous carbon portion of the fly ash absorbs the air entraining agents, at a higher rate, dramatically reducing their concentration in solution. The very fine carbon in the ash has a high surface area and behaves like activated carbon and is responsible for adsorbing the air entraining agents. Coarse carbon has a lower surface area and adsorbs less of the air entraining agents.
The amorphous carbon has a higher surface area and is more polar relative to crystalline carbon (R. Hill, S. Sarkar, R. Rathbone, J Hower, Cement Concrete Research, Vol.27(2),pp 193-204, 1997). The fraction of amorphous carbon in fly ashes varies from source to source, and can vary within the same source. The ASTM C618-01 Loss-On-Ignition method does not provide information on the amount of amorphous carbon in fly ash and therefore alternative methods, such as the ASTM C311 mortar method or foam index are required to determine the air entraining agent adsorbing affinity of the fly ash. The method proposed here is a modified foam index method.
The foam index method was originally developed by Dodson and further modified by Menninger (R. Helmuth, xe2x80x9cFly Ash in Cement and Concretexe2x80x9d, Portland Cement Association, 1987, pp 80-81). The original method consists of adding 50 ml of water to 16 grams of Portland cement and 4 grams of fly ash (Class F) in a 125 ml glass jar. The mixture is shaken vigorously for 1 minute and then a diluted air entraining agent, such as neutralized Vinsol resin, is added drop-wise, in increments of 1 to 5 drops. After each addition of air entraining agent solution, the jar is shaken vigorously for 15 seconds. The foam layer is observed for stability over a period of 45 seconds and if no significant collapse of the foam occurs then the number of drops of air entraining agent solution used is the foam index value. The foam index value corresponds approximately to the apparent saturation point where most of the active adsorption sites on the fly ash carbon are covered with air entraining agent molecules. If the slurry is left to stand and shaken again the foam stability will decrease with time, indicating that equilibrium has not been reached. Therefore, it is important to obtain the foam index value at approximately the same time and to limit the amount of time allowed for the determination.
For the purposes of the invention the foam method has been modified and consists of the following steps: 5 grams of fly ash is added to 70 ml. of distilled water containing 0.3 ml. 25% sodium citrate solution; the suspension is titrated with a solution of 0.2% sodium lauryl sulfate (SLS) solution until a stable foam layer is obtained after shaking vigorously 20 times after each incremental addition of sodium lauryl sulfate solution; the volume of the titrated solution is the value taken for the foam index. A foam index value of 0.8 ml was found to be the average value for commercially saleable Class F fly ash and the target for Partial Carbon Burn-Out.
The modified foam index method was developed for use with Class F fly ash which has an LOI between 2% and 5%. ASTM C618-01 limits the LOI value to a maximum of 6% (12% if concrete data is available showing that air can be entrained and that this level has been accepted by the user), but in many cases it is difficult to achieve and maintain the specified air contents in concrete containing Class F fly ash that has an LOI greater than 3%. Class C fly ash has an LOI in the range of 0.1 to 1.5%+ and ordinarily do not exhibit problems with air entrainment. Nevertheless, significant changes in air entrainment and stability problems are occasionally experienced with many Class C fly ashes. The usual indicators of fly ash quality, such as color and LOI, do not provide sufficient information on the quality of Class C fly ashes. The foam index method, on the other hand, can detect such changes where the fly ash is not noticeably darker, the LOI is normal, but the foam index indicates a high adsorption capacity. The thermal treatment of this invention does not substantially change the color of the fly ash product.